Wave,
I'll take a shot at this. If Delta had bought Airtran, I would think any arbitrator would have looked at the differences in the operations. They would look at the sizes of the planes, the difference in routes, and the number of pilots. Longevity would be a factor (years of service), too. If you look at the NIC award for USAir, he put the top 500 on the combined list as all East pilots because they flew INTL routes with A330s and 767s, both larger planes than America West's 757s. Delta is a lot larger than USAir, and there are more plane differences too, with even larger planes doing INTL routes. I would assume, using that recent precedent, that a large chunk of senior Delta pilots would have been put on top of a list, and then it would have maybe gone relative or a ratio from there. That is just a guess based off of the recent NIC award. Maybe Ty Webb (737 Capt) could have been placed somewhere in the Middle of the Delta 737 Captain list, which is probably mid level Captain seniority. He probably wouldn't have been blended in with any 757/767 Capts or above, and the junior 757/767 Captain in the fleet would also probably be placed above that 737 group. Just a guess there. The NIC award, putting the top 500 all from one group that had larger planes that flew INTL routes, really did set the bar.
Now when it comes to SWA and Airtran, your missions are the same, and the planes are very close to the same size. Airtran does fly International flights, (and allnighters), but isn't as large as Southwest. Southwest is more profitable, but Airtran isn't a money loser, and not in BK like Frontier was. That would mean there could be a ratio. But, pay doesn't seem very important to arbitrators (look at Colgan vs Pinnacle and Mesaba), and that might be significant in your case against Airtran. SLI's aren't fun, but they must be fair if you want to fly with these people eventually and not end up like USAir--a divided group. That will hurt you in future negotiations and that famous culture will just go away.
All of the above is speculation, of course.
Bye Bye--General Lee